As drilling tools used to drill into the ground, through sediment, and into the materials to be drilled, the drilling tools made up of drill bits having a plurality of button tips formed from cemented carbide and other hard materials, and drill rods to support the drill bits, are used. At the proximal end of a drill bit is formed a fitting hole having a female thread on the inner peripheral face. On the distal of a drill rod is provided a fitting portion having a male thread on the outer peripheral face. In a drill tool, a drill rod and a drill bit are formed into an integral unit by screwing the male thread of the fitting portion of the drill rod with the female thread of the fitting hole of the drill bit.
In recent years, demand for high-speed drilling execution has lead to requires for greater efficiency of drilling tasks through enhanced drilling equipment performance and increased drilling speed. However, using conventional drilling tools, the diameter of the fitting portion of drill rods is small in order to secure an adequate wall thickness for drill bits, so that when mounting such a drilling tool on drilling equipment and drilling through ground or similar, the load on the drilling tool is increased when the drilling speed is raised, and there are concerns that this load may cause damage to the drill rod. On the other hand, when the drill rod diameter is increased, the wall thickness of the drill bit is reduced, and so there is the problem that the drill bit may be damaged. In particular, when the drill rod breaks, the drill bit or similar remains at the bottom of the drilled hole, so that recovery entails considerable time and effort, which is uneconomical. As a result, there have been strong demands for improved rigidity of drill rods.
In Japanese Patent Document 1, an invention is proposed in which, by forming the fitting portion of a drill rod in a taper shape and using taper-shape threads for screwing, the wall thickness of the drill bit on the distal side of the drilling tool can be secured, while increasing the diameter of the drill rod on the drilling tool proximal side, at which the bending stress applied to the drill rod becomes large, so that rigidity of the drill rod and the drill bit are improved, and damage to the drilling tool due to the load is prevented.    [Patent Document 1] PCT International Publication No. WO 00/19056
However, in the case of the drilling tool disclosed in Patent Document 1, because the fitting portion is formed in a taper shape and screwing is performed using threads in a taper shape, there are concerns that shocks during drilling may cause the drill bit easily to be separated from the drill rod. Further, machining to form a taper-shape thread is difficult, and so there is the problem that the cost of manufacturing such drill rods and drill bits is increased.
Also, because a relief portion is formed in the distal end of a drill bit, that is, in the hole bottom portion of the fitting hole, in order to cut the female thread, a thin-wall portion is formed on the distal side of the drill bit, despite forming a taper-shape thread, and so there are concerns that the load on the drilling tool may cause damage to the drill bit.